One type of activated sludge sewage treatment plant combines several contiguous, endless aeration channels surrounding a single or several clarifiers. Raw waste, after screening and optional grit removal, is introduced into the first aeration channel where it is rapidly mixed into the system and circulated by rotating aeration discs. Besides mixing, the aeration discs supply the necessary oxygen for the activated sludge process. Although the aeration channels are complete mix systems, premature exiting of portions of the raw waste (resulting in lower BOD removal efficiencies) is prevented with two or more of the channels in series and with their transfer port located nearly full-circle from the feed to the first channel. Mixed liquor from the innermost aeration channel is usually introduced directly into the central final clarifier by a pipe or port which is similarly nearly full-circle from the transfer port.
This type of sewage treatment plant can particularly provide relatively economic but consistently high quality treatment in the flow range of 50,000 gpd to 350,000 gpd. In such plants the several endless aeration channels are circular and with the clarifier form one concrete structure. However, in the small sewage treatment plant the hydraulic design of the clarifier can become more critical than the organic load design and certain considerations are especially important in the design of the clarifier where the fluctuation of flow is great. As is known but not often noted in the design of small sewage treatment plants, the hydraulic design and the organic load design should be independent of each other. This is by comparison with the typical municipal treatment plant where the peak flows are from the ground runoff after a storm and add relatively little to the organic load. Heretofore, the usual clarifier has not been flexible or simple in operation, nor economical to build or to operate and will not give the utmost in treatment efficiency. The final clarifier in such a plant should provide a controlled flow pattern for optimum physical separation of solids at variable flow conditions, all at a minimum of capital costs. In particular also, an exceptionally simple means of scum removal from the clarifier must be provided.
An object of the present invention is to provide a completely non-mechanized clarifier with a minimum influent disturbance of the sludge blanket and with practically no flow short-circuiting.